The present invention relates to an apparatus, such as a solid electrolyte fuel cell, which includes opposing, symmetric, composite electrodes including a conductive metal phase and a ceramic phase and a method of making such an apparatus.
The use of solid electrolyte materials for fuel cells and oxygen pumps has been the subject of a considerable amount of research for many years. The typical essential components of a solid oxide fuel cell (xe2x80x9cSOFCxe2x80x9d) include a dense, oxygen-ion-conducting electrolyte sandwiched between porous, conducting metal, cermet, or ceramic electrodes. Electrical current is generated in such cells by the oxidation, at the anode, of a fuel material, such as hydrogen, which reacts with oxygen ions conducted through the electrolyte from the cathode.
Practical power generation units will typically include multiple fuel cells of such configuration interconnected in series or parallel with electronically conductive ceramic, cermet, or metal interconnect materials. At the present time, the materials of choice for such devices include yttria-(Y2O3) stabilized zirconia (ZrO2) for the electrolyte, nickel-ZrO2 cermet for the anode material, strontium-doped lanthanum manganite (LaMnO3) for the cathode, and metals, especially Cr/Fe alloys and Ni alloys, intermetallics, and Sr or Ba doped LaCrO3, for interconnect structures. Alternative oxygen ion conductors are known. At sufficient temperatures (e.g., 600xc2x0 C. or above), zirconia electrolytes can exhibit good ionic conductivity but low electronic conductivity.
Several different designs for solid oxide fuel cells have been developed, including, for example, a supported tubular design, a segmented cell-in-series design, a monolithic design, and a flat plate design. All of these designs are documented in the literature, with one recent description in Minh, xe2x80x9cHigh-Temperature Fuel Cells Part 2: The Solid Oxide Cell,xe2x80x9d Chemtech., 21:120-126 (1991).
A number of planar designs have been described which make use of free-standing electrolyte membranes. A cell is formed by applying electrodes to a membrane and consists of the electrolyte sheet and the applied electrodes. Typically these cells are then stacked and connected in series to build voltage. Monolithic designs, which characteristically have a multi-celled or xe2x80x9choneycombxe2x80x9d type of structure, offer the advantages of high cell density and high oxygen conductivity. The cells are defined by combinations of corrugated sheets and flat sheets incorporating the various electrode, conductive interconnect, and electrolyte layers, with typical cell spacings of 1-2 mm and electrolyte thicknesses of 25-100 microns.
U.S. Pat. No. 5,273,837 to Aitken et al. describes sintered electrolyte compositions in thin sheet form for thermal shock resistant fuel cells. It describes an improved method for making a compliant electrolyte structure wherein a precursor sheet, containing powdered ceramic and binder, is pre-sintered to provide a thin flexible sintered polycrystalline electrolyte sheet. Additional components of the fuel cell circuit are bonded onto that pre-sintered sheet including metal, ceramic, or cermet current conductors bonded directly to the sheet as also described in U.S. Pat. No. 5,089,455 to Ketcham et al. Advantages of the thin flexible sintered electrolyte structure include exceptional flexibility and robustness in the sintered state.
One requirement of fuel cell designs incorporating flexible ceramic and metallic layers is that of maintaining good thermal compatibility among the various electrolyte and electrode materials employed. For example, the use of a cathode material having a significantly higher thermal expansion coefficient than the anode introduces bending stresses or warpage in flexible fuel cell assemblies. While such bending can be tolerated without structural breakage if the electrolyte structure is thin, the need to accommodate the resulting shape distortions places limits on cell spacings and other aspects of geometric cell design.
Silver and its alloys are among the best electrical conductors known. Further, silver is both oxygen permeable and an excellent electrocatalyst for oxygen reduction. Therefore, silver has been used as a component in fuel cell cathodes despite its relatively high volatility at conventional fuel cell operating temperatures (800xc2x0 C. and above).
U.S. Pat. No. 5,395,704 to Barnett, for example, discloses a thin film, nickel-mesh supported fuel cell including a silver/yttria-doped zirconia cermet cathode. However, silver has not been employed to any degree as an anode material due to its relatively poor catalytic performance towards fuel oxidation in comparison with standard nickel-containing anode compositions.
The present invention is directed to providing an improved fuel cell construction, applicable to any of the above fuel cell designs, which avoids many of the difficulties of fuel cell manufacture while providing a cell of improved physical, thermal, and electrical properties. In particular, the present invention is directed to high performance electrodes for intermediate-temperature solid oxide fuel cells.
The present invention relates to devices such as electrode/electrolyte assemblies for fuel cells which include a positive air electrode or cathode, a negative fuel electrode or anode, and a ceramic electrolyte structure interposed between and supporting the positive air electrode and the negative fuel electrode. The positive and negative electrodes are symmetric in that they are composite electrodes of similar base composition, with similar physical and thermal properties. They both comprise a conductive metal phase, typically a silver alloy, and a ceramic phase for improved electrode stability at high temperatures.
In order to develop good fuel oxidation activity in the cell, the negative (fuel) electrode is provided with a catalyst addition to enhance the fuel oxidation activity of the silver-based electrode. The catalyst is added in a proportion at least effective to increase fuel oxidation activity but not to substantially change the physical properties of the electrode.
The present invention also relates to a solid oxide fuel cell which includes a plurality of such assemblies, each comprising a positive air electrode and a negative fuel electrode formed of a composite of a conductive silver-containing metal phase and a ceramic phase, the electrodes being disposed on and supported by a ceramic electrolyte structure interposed between the electrodes.
Another aspect of the present invention relates to a method of making an electrical device which involves providing a thin, flexible ceramic electrolyte substrate and applying on opposing sides of the electrolyte substrate in symmetrical fashion thin electrode layers including a conductive metal phase and a ceramic phase.
The use of appropriate ceramic components in the silver-containing electrodes of the present invention reduces electrode interface resistance and improves electrode durability. In particular, electrodes so comprised display exceptionally low ohmic and interfacial resistance for both the air side (cathode) and fuel side (anode). Moreover, fuel cell devices of the present invention show excellent tolerance towards leakage of fuel into the air chamber or air into the fuel chamber. Such leakage may be expected in practice due to the presence of pinholes through the electrolyte, or egress through seals.
The superior electrical conductivity of the composite silver alloy electrodes allows use of extremely thin electrodes, as a thickness of only a few microns is needed to achieve acceptable ohmic loss. Assemblies provided by combining thin, flexible self-supporting electrolyte sheets with thin silver alloy electrodes are quite flexible and hence extremely thermal shock tolerant.
Further, notwithstanding the flexible character of the electrolyte sheet, the thermal expansion match between the anode and cathode layers results in a stress field symmetry that produces a flat, composite assembly. In assemblies based on flexible electrolytes, thermal mismatch within a bi-layer electrode/electrolyte structure is manifested as warpage of the bi-layer body. However, in tri-layer structures (anode/electrode/cathode), if the anode and cathode have closely matched thermal expansion behavior, the stress field is symmetric and a flat body is maintained. Maintaining an overall planar electrode/electrolyte assembly is important for low cost fuel cell stack fabrication.